Protein kinases (PKs) are enzymes that catalyze the phosphorylation of hydroxy groups on tyrosine, serine and threonine residues of proteins. Protein kinases, and in particular the receptor protein tyrosine kinase (RTK) family of protein kinases, act primarily as growth factor receptors and play a central role in signal transduction pathways regulating a number of cellular functions, such as cell cycle, cell growth, cell differentiation and cell death. Aberrant or excessive activity or the disregulation of activity of receptor protein tyrosine kinase (RPTK) has been observed in many disease states including benign and malignant proliferative disorders as well as inflammatory disorders and immune system disorders that result from inappropriate activation of the immune system to cause, for example, autoimmune diseases.
Disregulated activity of the receptor tyrosine kinase of the platelet growth factor receptor (PDGFR) family, as one example, has been implicated in various proliferative disorders. Gene amplification or upregulation of PDGFR occurs in patients with gliomas or sarcomas (Kumabe et al., Oncogene, 7:627-633 (1992), Ostman and Heldin Cancer Res. 80:1-38 (2001)). Constitutive activation of PDGFR-α has been found in patients with chronic myelomonocytic leukemia (CMML) (Magnusson et al. Blood 100:1088-1091 (2002)). Gain of function mutations and small deletions in the PDGFR-α gene has also been found in patients with gastrointestinal tumors (GIST) (Heinrich et al. Science 299: 708-710 (2003)) and in patients with idiopathic hypereosinophilic syndrome (Cools et al. N. Engl. J. Med. 348:1201-1214 (2003)). PDGFR-β has been found to be expressed in the tumor stroma in a majority of solid tumors, which makes this receptor a potential target for anti-tumor therapy (Pietras et al. Cancer Cell 3:439-443 (2003), Pietras et al. Cancer Res. 62: 5476-5484 (2002)). PDGFR-β has also been found to be expressed in tumor vasculature and studies have suggested PDGFR-β inhibition as one mechanism for anti-angiogenic therapy. (See, Bergers et al J. Clin. Invest. 111(9): 1287-1295 (2003), Saharinen et al. J. Clin. Invest. 111:1277-1280 (2003)).
A second member of the PDGFR family, Flt3 (also called Flk2), plays an important role in the proliferation and differentiation of hematopoietic stem cells and activating mutation or overexpression of this receptor is found in AML (See, Heinrich Mini-Reviews in Medicinal Chemistry (2004) 4(3):255-271, Kiyoi et al. Int J Hematol (2005) 82:85-92). More than a dozen known Flt3 inhibitors are being developed and some have shown promising clinical effects against AML (See Levis et al. Int J Hematol. (2005) 82:100-107). The Flt3 receptor is also expressed in a large portion of dendritic cell progenitors and stimulation of the receptor causes the proliferation and differentiation of these progenitors into dendritic cells (DC). Since dendritic cells are the main initiators of the T-cell mediated immune response, including the autoreactive immune response, Flt3 inhibition is a mechanism for downregulating DC-mediated inflammatory and autoimmune responses. One study shows the Flt3 inhibtor CEP-701 to be effective in reducing myelin loss in experimental autoimmune encephalomyelitis (EAE), a mouse model for multiple sclerosis (See Whartenby et al. PNAS (2005) 102: 16741-16746). A high level of the Flt3 ligand is found in the serum of patients with Langerhans cell histiocytosis and systemic lupus erythematosus, which further implicates Flt3 signaling in the disregulation of dendritic cell progenitors in those autoimmune diseases (See Rolland et al. J. Immunol. (2005) 174:3067-3071).
A third member of the PDGFR family, colony-stimulating factor-1 receptor (CSF-1R) (also called macrophage colony stimulating factor receptor (M-CSFR) or fms) is expressed by many carcinomas of the breast and human epithelial cancers, especially of the female reproductive tract (Kacinski (1997) Mol. Reprod. Dev. 46:71-74) and presents a potential target for cancer therapies. High level of CSF-1 expression in solid tumors and leukemias, also suggests that CSF-1R might be a therapeutic target for blood cancers and solid tumors (Haran-Ghera (1997) Blood 89:2537-2545). A high level of CSF-1 expression is also found in Langerhans cell histiocytosis (Rolland et al. J. Immunol. (2005) 174:3067-3071).
Kit (or stem cell factor receptor, or SCFR) is another member of the PDGFR family, and the presence of kit mutations is a key diagnostic marker for gastrointestinal stromal tumors (GIST) (Duensing et al. (2004) Cancer Investigation 22(1):106-116). Gleevec® (imatinib mesylate or STI571), the first FDA-approved RPTK inhibitor originally approved for c-Abl-mediated chronic myeloid leukemia, gained FDA-approval for Kit-mediated GIST in 2002 and has validated the molecular-based approach of Kit inhibition for the treatment of GIST. (Giorgi and Verweij, Mol Cancer Ther 4(3):495-501 (2005)). Gain of function mutations of the Kit receptor are also associated with mast cell/myeloid leukemia and seminomas/dysgerminomas (Blume-Jensen Nature 411(17): 355-365 (2001). Kit mutations have been also identified in certain melanomas and recognized as a potential therapeutic target for melanoma (Curtain et al. J. Clin. Oncol. 24(26):4340-4346 (2006)).
The vascular endothelial growth factor receptor (VEGFR) represents another family of RTKs, one that is implicated in tumor angiogenesis. VEGF and its receptors VEGFR1 (also called Flt1) and VEGFR2 (also called KDR) are overexpressed in the great majority of clinically important human cancers including cancers of the gastrointestinal tract, pancreas, bladder, kidney, endometrium and in Kaposi's sarcoma. VEGFR2 is also highly expressed in certain intracranial tumors including glioblastoma multiforme and sporadic and von Hippel Landau (VHL) syndrome-associated capillary hemangioblastoma. There are currently more than a dozen VEGFR2 inhibitors in clinical development for anti-angiogenic therapy (Paz and Zhu, Frontiers in Bioscience 10:1415-1439 (2005)).
Another member of the VEGFR family, VEGFR3 (also called Flt 4) has been identified as a lymphangiogenic growth factor receptor which play a key role in the growth of new lymphatic vessels (lymphanigiogenesis). Activation of the VEGFR3 signaling pathway has been shown to stimulate metastatic spread of tumor cells (See Stacker et al. Nature Rev 2:573-583 (2002)) and therefore its inhibition could be the basis for treating conditions characterized by abnormal lymphatic vessel function (See Stacker et al. Current Pharmaceutical Design 10:65-74 (2004), Achen et al. British Journal of Cancer 94:1355-1360 (2006)).
Ret kinase is yet another RTK, one that is found expressed in medullary thyroid carcinoma, a condition that is part of the multiple endocrine neoplasia 2A and 2B (MENS 2A and 2B) syndromes. Ret is constitutively active in medullary thyroid carcinoma (both familial and sporadic) and in papillary thyroid carcinoma. Some known RTK inhibitors having Ret-inhibitory activity have been shown to be effective in inhibiting tumor growth in nude mouse models (Stock et al., Cancer Res 63:5559-5563 (2003)) and Carlomagno et al., Journal of the National Cancer Institute 98(5):326-334 (2006)).
It is additionally possible that inhibitors of certain kinases may have utility in the treatment of diseases when the kinase is not misregulated, but is nonetheless essential for maintenance of the disease state. In such cases, inhibition of the kinase activity would act either as a cure or palliative for these diseases. For example, many viruses, such as human papilloma virus, disrupt the cell cycle and drive cells into the S-phase of the cell cycle (Vousden, FASEB Journal, 7:8720879 (1993)). Preventing cells from entering DNA synthesis after viral infection by inhibition of essential S-phase initiating activities, may disrupt the virus life cycle by preventing virus replication. This same principle may be used to protect normal cells of the body from toxicity of cycle-specific chemotherapeutic agents (Stone et al., Cancer Research, 56:3199-3202 (1996); Kohn et al., Journal of Cellular Biochemistry, 54:44-452 (1994)).
Finally, while overactivation of RTK signaling pathways is often the underlying mechanism for cancer, impaired deactivation of RTKs such as the impaired down-regulation of RTKs via ligand-induced endocytosis or impaired negative feedback loops, may also be the cause of some malignancies. Another strategy for use of the molecules discussed herein therefore is to repair and promote any existing mechanism for down-regulating RTKs.
In view of the large number of protein kinase inhibitors and the multitude of PK-mediated proliferative, inflammatory and immune function diseases, there is an ever-existing need to provide novel classes of compounds that are useful as PK inhibitors and thus in the treatment of PK related diseases, as discussed herein.